Process for the manufacture of articles which are made of iron or steel and are threaed or possess surfaces which make tight joints



Patented Mass, 1941 I f 2,236,209

PATENT OFFICE 2,236,209 I I PROCESS FOR THE MANUFACTURE OF AR- TICLES WHICH ARE MADE OF IRON OR STEEL AND ARE THREADED OR POSSESS SURFACES WHICH MAKE TIGHT JOINTS I Karl Daeves'and Gottfried Becker, Dusseldorf Germany.

No Drawing. Application May 10, 1938, Serial No.

207,070. In Germany May 11, 1937 1 Claim. (Cl.- 148-14) For vessels in which operations are carried out larly favorable results in the case of screws, nuts, at a high temperature with or without the apclosure plugs and other closing and tightenin plication of high pressure, bolts, screws, nuts and orv connecting elements which are made of low the like made of heat-resistant and corrosionand medium alloysteels, for example of chro- 5 resistant steel alloys having a low carbon content mium-molybdenum steels, which may also con- 5 have hitherto been employed. As is well known tain additions of nickel, vanadium, tungsten and the heat-resistant and corrosion-resistant alloy so forth; whereas all such articles are likely to steels are very likely to seize so that screw conseize to the parts to which they are connected nections or parts'used in making a tight closure after remaining for even only a short time. at a cannot be released later or can be released only temperature of 500 C. if they are not trea d 10 with considerable difliculty. This tendency of with'a gaseous chromium-containing medium in alloy steels to 'seize is particularly troublesome accordance with the invention. If threaded in the case of connecting elements and tightenparts, closure surfaces and the like are treated ing parts used in high pressure apparatus which I in the manner described then they can be released must fit withaclearance of onlyafew hundredths from the parts to which they are connected 15 of a millimeter. In such cases it frequently hapwithout any difflculty even when the vessel to pens that a nut seizes on being screwed upon the which they are attached or in which they are bolt or the screw connections of the cover can inserted has been in continuous use at temnot be released after the vessel has been in use peratures of 600 C. and higher. The highly for only a short time. In the case of connecting alloyed ferritic and martensitic heat-resistant and 20 bolts provided with screw threads and also of corrosion resistant steels, which are otherwise packing parts which lw ed to high temalso inclined to seize even at a low pressure, lose peratures, for example, 400 C. and higher, the this property when they are treated in chromium difliculties are still more pronounced owing to chloride gas at a temperature above 850 C.

the occurrence of superficial oxidation. The parts provided with screw threads and 25 It has now been found that the troublesome tightening or closure surfaces which have been seizing of threaded parts made of iron or steel treated in accordance with the invention at temalloys, such as nuts, screws and'the like, and of peratures of about 850 C. or higher for example surfaces by which tight contact is made, espein a current of chromium chloride gas, acquire UNITED STATE y se W ic are subjected to hi pressure an extremely firmly'adherent surface layer, which 30 and high temperature, can be entirely eliminated consists of an iron-chromium alloy, and is such bytreating the parts or surfaces in a chromiumthat there is a gradual transition to the subcontaining gas, preferably chromium'chloride gas stance of the object. By'this gradual transition (Crew, and thereby enriching the r Su ace w th this layer is clearly distinguishable from chro- 5 chromium. The enrichment with chromium is mium-containing superficial coatings which are 3-) I effected, in, accordance with the invention, at produced electrolytically. This result is of decimp r es f pp xim y n hi hsive importance in the present case, because a either y packing he articles in a ediu superficial layer which flakes or cracks of! on the for example a chromium chloride salt which slightest deformation owing to cold or heat is 40 evolves achromium-con ain n gas or, as has more harmful than useful on threads and sur- 40 provided particularly suitable, by conducting a faces, which makeatight joint. I current of chromium chloride gas over the parts Since the surface layer which is formed in or articles which have been heated to the stated accordance with the invention by treatment of temperature. Q articles in chromium chloride gas at a, tempera- The surface of articles whichhave been treated ture of about 850 C. 0r higher adheres extremely 45 in this manner is converted into an extremely firmly to the articles and is actually combined dense and smooth iron-chromium alloy owing to with the remaining material, it can be. applied the fact that by the diffusion of chromium in the to articles or parts made of iron or steel alloys form of gas, especially chromium chloride gas at even before they have been finally finished.

a temperature above 850 C., the chromium con- Thus, for example, a bolt made of iron or a steel tained in the gas is exchanged for an equivalent alloy which is to be provided with a screw thread .proportion o1 the iron in the objects treated. can be first roughly shaped and then enriched which, at the same time, maintain substantially with chromium on its surface by treatment at a their original dimensions. temperature above 850" C., and it can then be .The aforesaid surface treatment has particufinished by bringing it to the exact dimensions required and providing it with the thread. The

parts which have been superficially enriched with chromium in accordance with the invention can be subjected to considerable deformation by cold or hot treatment because the gaseous diffusion of chromium which takes place atan elevated temperature produces-a surface layer which is so combined with the remainder of the material that cracking or flaking off of the surface layer by later hot or cold working need not be feared. It is therefore also possible subsequently to machine the parts which are made of iron or steel alloys and have been treated in gaseous chromium carriers at temperatures'of above 850 C. and to work themsubsequently by a hot or cold deformation treatment in any desired manner. when the objects are exposed to varying temperatures, as frequently occurs with connecting parts, the surface layer is not impaired in any way.

Even

We claim: The process of preventing seizing of closely interfitting parts made oi iron alloy, when they are stationary relative to one another and maintained tightly held together, while being subjected to high temperature and corroding influence, said process consisting in modifying the outermost portion of at least one of said parts by chemically replacing some of the iron therein by chromium, by subjecting said part to the action 01' a gaseous chromium chloride at a temperature above 850 0., thus producing an article of substantially unaltered dimensions, which superficially consists of a chromium-iron alloy intimately united with the iron alloy beneath it.

KARL DAEVES. GO'I'I'F'RIED BECKER. 

